1. Field of the Invention
The present invention relates to a refrigerator and is more particularly concerned with a cryogenic refrigerator having improved refrigerating capacity.
2. Discussion of Background
FIG. 10 is a schematic diagram showing the structure of a conventional cryogenic refrigerator which has been disclosed in e.g. Japanese Examined Patent Publication No. 30433/1971 (U.S. Pat. No. 3,281,815). The conventional cryogenic refrigerator is a refrigerator having the Gifford-McMahon cycle. In FIG. 10, reference numeral 1 designates an operating gas (for example helium gas). Reference numeral 2 designates a suction valve for sucking the operating gas 1. Reference numeral 3 designates an exhaust valve for exhausting the operating gas 1. Reference numeral 4 designates a first step expansion chamber. Reference numeral 5 designates a first step displacer as a movable member which reciprocates to move the operating gas 1. Reference numeral 6 designates a first step cold accumulator which is used to accumulate the cold in the operating gas. Reference numeral 7 designates a first step seal which is used to prevent the operating gas 1 in the first step expansion chamber 4 from leaking along the outer periphery of the first step displacer 5. Reference numeral 8 designates a first step refrigerating stage which is used to transfer the cold in the first step expansion chamber 4 to outside. Reference numeral 9 designates a first step cylinder. Reference numeral 10 designates a second step expansion chamber. Reference numeral 11 designates a second step displacer as a movable member which reciprocates to move the operating gas 1. Reference numeral 12 designates a second step cold accumulator which is used to accumulate the cold in the operating gas. Reference numeral 13 designates a second step seal which is used to prevent the operating gas 1 in the second step expansion chamber 10 from leaking along the outer periphery of the second step displacer 11. Reference numeral 14 designates a second step refrigerating stage which is used to transfer the cold in the second step expansion chamber 10 to outside. Reference numeral 15 designates a second step cylinder. Reference numeral 16 designates an electric motor which drives the displacers 5 and 11. Reference numeral 17 designates a driving shaft which is used to transmit a driving force from the electric motor 16 to the displacers. Reference numeral 18 designates a crankshaft for converting the rotational movement of the motor into a reciprocating movement for the displacers. Reference numeral 19 designates a compressor for compressing the operating gas 1. Reference numeral 20 designates a high pressure buffer tank which can minimize variation in the pressure at a higher pressure side. Reference numeral 21 designates a low pressure buffer tank which can minimize variation in the pressure at a lower pressure side. Reference numeral 22 designates a device for maintaining at a constant level the difference in the pressures at the higher pressure side and at the lower pressure side. An arrow 23 designates an amount of refrigeration Q1 which is absorbed by the first step refrigerating stage 8. An arrow 24 designates an amount of refrigeration Q2 which is absorbed by the second step stage 14. The operation of the cryogenic refrigerator will be explained. FIG. 11 is a P-V diagram of the refrigerator. The ordinate represents the pressures in the first step expansion chamber 4 and the second step expansion chamber 10, and the abscissa represents the volumes in the both chambers. Under the condition at A in FIG. 11, the first step displacer 5 and the second step displacer 11 are at their lowermost positions, and the suction valve 2 and the exhaust valve 3 are opened, causing the pressures in both chambers 4 and 10 to become high. In the course of A-B, the displacers 5 and 11 are raised, causing the operating gas 1 having high pressure to be introduced from the compressor 19 into the expansion chambers 4 and 11 while being cooled in the cold accumulators 6 and 12. The cold accumulators 6 and 12 have such temperature gradients that the temperature at the upper end of the first step cold accumulator 6 is e.g. 300 K, the lower end of the first step cold accumulator is e.g. 50 K, the upper end of the second step cold accumulator 12 is e.g. 50 K and the lower end of the second step cold accumulator is e.g. about 10 K. In this case, the operating gas 1 which has been introduced into the first step expansion chamber 4 is cooled to about 50 K, and the operating gas 1 which has been introduced into the second step expansion chamber 10 is cooled to about 10 K. The volumes in the expansion chambers become maximum at B. At this time, the cold accumulators have temperature distributions which are at higher levels than their initial temperature distributions because the cold accumulators have been heated by the operating gas 1. In the course of B-C, the suction valve 2 is closed while the exhaust valve 3 is opened. In this course, the operating gas 1 is expanded to change from a high pressure state to a low pressure state to generate cold in the expansion chambers 4 and 10. The principle of this cold generation is indicated in FIG. 12. Firstly, the operating gas 1 having high pressure which is in the second step expansion chamber 10 under the condition B is imaginarily divided in x1 to x7. When the exhaust valve 3 is opened, the portion x1 of the operating gas 1 flows out to achieve the condition of b1. As a result, the portions x2 to x7 of the operating gas 1 expand, causing the temperature of the operating gas to lower. Next, the portion x2 of the operating gas 1 flows out to achieve the condition of b2. As a result, the portions of x3 to x7 of the operating gas 1 expand, causing the temperature of the operating gas to be further lowered. Such process is repeated, leading to the condition of C. The change from the condition of B to the condition of C is substantially an adiabatic change because the change from the condition of B to the condition of C instantly occurs and heat transfer with the second step refrigerating stage 14 is poor. The operating gas 1 thus expanded receives at the first step refrigerating stage 8, the amount of heat which is a portion of the amount of refrigeration Q1, and also receives, at the second step refrigerating stage 14, the amount of heat which is a portion of the amount of the refrigeration Q2. Next, the operating gas 1 cools both cold accumulators 6 and 12, and then returns to the compressor 19. At the condition of C, the pressures in the expansion chambers 4 and 10 are low. In the course of C-D, the displacers 5 and 11 move downward to exhaust the operating gas 1 whose pressure has lowered. The expanded operating gas 1 which is exhausted in this course also receives, at the first step refrigerating stage 8, the amount of heat which is the remaining portion of the amount of refrigeration Q1, and further receives, at the second step refrigerating stage 14, the amount of heat which is the remaining portion of the amount of refrigeration of Q2. The operating gas 1 cools the cold accumulators 6 and 12, and then returns to the compressor 19. In the course of D-A, the exhaust valve 3 is closed while the suction valve 2 is opened, causing the pressures in the expansion chambers to change from the low level to the high level. In this way, one cycle is completed. In the course of B-D, the cold accumulators 6 and 12 are cooled to recover the temperature distribution which is similar to that at the beginning of the cycle.
Since the conventional cryogenic refrigerator is constructed as above-mentioned, the change from B to C is an adiabatic change, causing the amount of refrigeration to decrease. In addition, heat transfer with the refrigerating stages 8 and 14 is not enough, the operating gas 1 enters into the cold accumulators 6 and 12 having temperature gradients, with the operating gas 1 being kept cold. That creates a problem wherein generated cold can not be fully utilized and refrigeration efficiency lowers. In particular, the loss at the second step refrigerating stage 12 introduces a problem.